Optical Feedback for Sensitivity Enhancement in Direct Raman Detection of Liquids

Abstract

Detection of low-concentration molecules in liquids has been a challenge in sensing technologies. Raman spectroscopy is an effective approach for trace detection, which is in fact a “volume-excitation” and “volume-collection” technique in the analysis of liquid samples. However, for the commonly employed one-pass excitation and back-scattering detection scheme, a large portion of both the excitation laser energy and the Raman-scattering light energy is wasted without efficient reuse or collection. In this consideration, we demonstrate a broadband optical feedback scheme by a curved high-reflection mirror for both the excitation and the Raman-scattering light, so that the excitation and the forward-propagating Raman signal can be back-reflected and collected with a high efficiency. Using the “F+2f” design, where F and f are the focal lengths of the focusing lens and curved reflection mirror, respectively, we were able to not only produce two focuses of the excitation laser beam but also extend the Raman interaction by a doubled distance. For the detection of pure ethanol molecules and the R6G molecules in water with a concentration of 10−3 M, the Raman signal was enhanced by a factor of about 5.6. The optical feedback scheme and discovered optical mechanisms supply effective improvements to the Raman spectroscopic measurements on liquid samples.

Quartz Tube Enhanced Raman Scattering Spectroscopy

“…With the most significant advantages such as simplicity, high chemical specificity, multicomponent and nondestructive detection, spectroscopy has been used in many fields [4], e.g., food adulteration [5], hazardous chemical detection [6], and drug analysis [7]. However, the Raman-scattering signal was very weak, which has an intensity lower than (10 -6 ) that of the excitation [8]. Therefore, approaches to enhance Raman scattering signals are always important to improve and realize related detection techniques.…”